1. Field of the Invention
The present invention relates to an image generation apparatus which generates an image, for example, in an three-dimensional graphic system, by mapping images which have been prepared in advance (texture images) onto polygons generated by a drawing operation with a brightness calculated for each polygon.
2. Description of the Related Art
In a three-dimensional graphic system, for example, an object is displayed as an aggregate of a multiplicity of polygons, and each of the polygons can be modified by a texture image prepared in advance. Such modification of polygons is achieved by performing a process called texture mapping in which texture images are applied to polygons. During such texture mapping, in order to give the displayed object a three-dimensional appearance, a so-called shading process is carried out in which the position of a virtual light source is set and each polygon is shaded in relation to the light source. Such shading is carried out by assigning a shading constant K=1 (i.e., the brightness of the original texture image) to the brightness of the polygon in the position where the highest brightness is obtained from the virtual light source and by setting the shading factors K for polygons darker than that to values which are smaller than 1 and which depend on how dark they are.
Conventionally, texture images prepared for texture mapping have been rendered using the entire dynamic range for representing the brightness of the image display device. As a result, the brightness of polygons can be set only in the direction in which it is reduced during texture mapping. Accordingly, it has not been possible to perform texture mapping so that texture images are mapped with a brightness higher than that of the original color thereof.
Therefore, considering the necessity to render an image with a brightness higher than the original normal brightness as encountered in rendering the scene of an explosion, it has been necessary to prepare texture images with a higher brightness than the normal brightness. An image having a normal appearance (image having a normal brightness) has been generated by mapping texture images of a higher brightness as described above so that the brightness is reduced.
The applicant has made the following Japanese patent applications which are related to the drawing device according to this application.
05-190763 (filed on Jun. 30, 1993) PA0 05-190764 (filed on Jul. 2, 1993) PA0 05-258625 (filed on Oct. 15, 1993) PA0 06-027405 (filed on Jan. 31, 1994)
Each of the above applications is owned by the assignee of the present invention and is hereby incorporated herein by reference. (Applications for U.S. patent corresponding to these four Japanese patent applications are pending.)
As described above, for conventional texture mapping, texture images having a brightness higher than a normal brightness are created in advance in order to render an image with an increased brightness. Therefore, it is not possible to create texture images having a normal appearance. This has resulted in a difficulty in creating texture images. There is another problem in that the creation of texture images takes a long time.
For example, an image having a normal brightness created using texture mapping is generated by reducing the brightness of the texture images of a higher brightness prepared in advance. This makes it difficult to render an image with a natural normal brightness because such a process renders even an image having a normal brightness with a darker appearance.
The present invention confronts the above-described problem, and it is an object of the present invention to provide a method and apparatus for generating an image which make it possible to prepare texture images of a normal brightness and to generate a texture-mapped image as an image brighter than the original texture images.